Electron gun



Aug. 15, 1950 o. HEIL ELECTRON cum Filed Feb. :5. 1949 INVENTOR. 05,608 5/;

BY [ULAM [jg/Q1755 Patented Aug. 15, 1950 ELECTRON GUN Oskar Heil, Konstanz, Germany Application February 3,1949, Serial N 0. 74,283

8 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to apparatus for the production of an electron beam.

It is the object of the invention to provide an electron gun capable of producing a parallel, well defined beam of electrons of high current density and highuniformityof cross-section, and having a cross-sectional area much smaller than the area of the. emitting surface of the cathode.

The gun may be used wherever a well defined high intensity beam of electrons is desired, several examples of which will be given later. The use of the gun in a novel power amplifier forms the subject matter. of my application Serial No. 74,282 filedFebruary 3, 1949.

The details of the electron gun design and structure will be given in connection with the accompanying drawings in which:

'Fig. 1 is a cross-sectional view of the electron un and Figs. 2 and 3 show possible methods of supporting the cathode of the electron gun.

The electron gun consists of two specially shaped electrodes, namely, a cathode with emitting and non-emitting parts and an additional electrode. The configurations and relative positions of the cathode and additional electrode are responsible for the desirable characteristics cf the gun, and this information is therefore given in detail in Fig. l. The dimensions in this figure are relative only. The absolute size of the Whole arrangement can be changed without disturbing the value of the current in the beam, which-obeys the 3/2 power law, or. the ratio between the area of the emitting surface of the cathode and the crossesectional area of the beam, which is of -the order of 230. However, for a given anode voltage, the current density in the beam is decreased as the size of the system is increased. A practical size of the gun may be obtained by letting the dimensional figures in Fig. 1 represent millimeters.

Referring to Fig. l, the emitting surface of the cathode is designated l and is a hollow surface the curvature of which is lowest at the center and increases toward the border. More precisely the surface shown is that of an ellipsoid of rotation in which the ratio of the major axis of the rotated ellipse to the minor axis thereof is 1.3:1. As-shown this surface has a depth of 6.15 and a maximum'diameter of 18.6,.these figures being relative .only's-as already pointed out. The surface the positive electrode 4.

I may be heated in any suitable way'to provide electronemission therefrom. The non-emitting part of the cathode comprises non-emitting surfaces 2 and 3. Surface 2 is that of a cone having an apex angle of 225. -'Thenon emitting-surface 3 is aplanesurface at right angles to the axis of symmetry 5 and extends inwardly toward -Theelectrodes I and 2-3 are always maintained at the same electrical potential, howeverjit maybe necessary to thermally insulate thetwoas will be seen later.

The electrode i has a toroidalsurface, with a radius of 3, tangentially joined to a conical surface in which the apex angle is 40. This electrode substantially fills the opening inportion 3 of the non-emitting part of the cathode. Electrode 4 is maintained at some positive potential relative of the cathode, however, the exact value of this voltage is not critical.

With the above described configuration and relative positioning of the electrodes the boundary of the electron stream is shown by the dotted lines in Fig. 1. The beam leaves the gun along parallel lines and is of high intensity, the crosssectional area thereof being approximately /230 of the area of emitting surface i. (not shown) potential of 1000 volts the current in the beam is milliamperes.

The above described electron gun has a number of uses as, for example, in Klystrons, travelling wave tubes and other tubes for the product-ion or amplification of short waves. The advantage here as compared with known arrangements is in the high current density and high quality of the beam with parallel electron travel and uniform intensity over the cross-section.

Another use for the electron gun is in television tubes, especially projecting tubes. Advantages here are: (1) high current density; (2) n0 distraction of the emitting surface by ion bombardment since ions entering the system are focussed by electrode 3 on a small spot in the center of the cathode, whereas the electronscome from the whole emitting surface; and (3) the focal point of the electrons is fixed in space and therefore modulation of the electron stream, by applying a modulating voltage to electrode 4, does not change the best focussing on the screen. In known systems controlling the beam intensity changes the focal point. This difficulty is entirely avoided by use of the new gun in a television tube.

Further examples of uses of the gun are in X-ray tubesfor making ultraspeed radiographs using very-short exposure times and high cur- With an anode I 3 rents, and in high intensity incandescent light sources especially for projecting purposes as in cinema projection. In the latter case the beam is focussed on an incandescent material of high melting point and low volatility as, for instance, tantalum carbide.

Another application of the electron gun is the high temperature treatment of surfaces of solid bodies. A solid body brought into the focal point of the electron gun can be heated in a very short time second) to temperatures of several thousand degrees. A very thin layer of about 0.01 millimeter is heated and then cools very quickly due to the small heat content and the high conduction into the solid body. By driving the gun continuously with short intense pulses and slow movement of the sample a large surface can be temperature treated. This temperature treatment may be used to obtain an extremely fine crystalline structure at the surface of a material by melting a thin layer of the material at the surface. This process may alsobe used for coating a material with a substance having a higher melting point than the ground material as, for instance, steel on aluminum or tungsten on copper. The coating material can be brought onto the surface by galvanizing, by evaporation, by cathodesputtering or by spraying and is then melted by the electron beam in which case alloying or chemical reaction with the ground material is avoided by the quickness of the action. The rapid heating action of the high intensity electron beam may also be made use of in alloying materials which could not be alloyed in a process requiring considerable time due to the volatility of one of the components.

Referring again to the electron gun structure in Fig. 1, it was pointed out that portionv i of the cathode is emitting whereas portion 2 is thereof is non-emitting. In a practical embodiment of the gun two ways of preventing emission from the non-emitting part may be employed. One is to keep the non-emitting part cold by having it thermally isolated from the emitting part; the other is to have the non-emitting part at the same temperature as the emitting part but to use a material having a higher work function than that of the emitting part. The first method is to be preferred for big cathodes because it saves heating power and the second method is preferable with small cathodes, as in television tubes.

A simple way of providing thermal insulation between the emitting and non-emitting portions of the cathode and at the same time having a very small'distance between the hot and cold parts is shown in Fig. 2. It uses the thermal expansion of'the emitting part to produce a distance between the two parts which in the cold state touch each other. The emitting portion I of the oathode is backed by a suitable electrical heating element and cover therefor, all forming a unitary structure 20. This structure is supported on the extended cold part 2-3 by inclined legs h) and If and two similar legs at right angles thereto only leg 2! of which is shown. In manufacture the legs are welded to extend electrode 2-3 and to the unitary emitting electrode structure 23 at points 22, 23, 2t and a fourth point not shown, With'the emitting electrode 20 resting on and touching the non-emitting electrode 2-3. When the emitting electrode structure 25 is heated, however, the distances between the fix points 22 and 23 and between the fixed points 2 3 and an opposite point not shown increases'slightly due to thermal expansion of the structure 29 and this 4 causes the hot portion of the cathode to lift off the cold part thus preventing direct heat conduction to cold part 2-3.

Fig. 3 shows a modification of the method used in Fig. 2. The heated part of the cathode is in this case supported by crossed inclined legs l4, l5, l8 and I1 mounted on conical extension it of cold part 2-3. The principle of operation is the same as for Fig. 2, expansion of the unitary emitting structure 20 causing the distances between points 22' and 23 and between points 24' and an opposite point, not shown, to increase slightly thus lifting the hot emitting electrode structure 20 off the cold part 23.

I claim as my invention:

1. An electron gun comprising a cathode having a hollow emitting surface the curvature of which increases. from the center to the edge thereof, an additional electrode positioned opposite said emitting surface and having a passageway therethrough converging in a direction away from said surface, and means for maintaining said additional electrode at a fixed potential relative to said cathode, whereby electrons released by the emitting surface of said cathode pass through said passageway to form a beam having a cross-sectional area much smaller than the area of said emitting surface.

2. An electron gun comprising a cathode made up of electron emitting and non-emitting parts, said emitting part having a hollow emitting surface the curvature of which increases from the center to the edge thereof, said non-emitting part having an edge conforming to the edge of said hollow emitting surface and positioned adjacent thereto whereby said non-emitting part forms a closure for said emitting surface, an opening in said non-emitting part opposite said emitting surface, an additional electrode positioned within said opening and having a passageway therethrough, said passageway having its longitudinal axis of symmetry perpendicular to said emitting surface and having a convergence in a direction away from said emitting surface, and means for maintaining the emitting and non-emitting parts of said cathode at the same electrical potential and at a lower potential than that of said additional electrode, whereby electrons emitted from said hollow emitting surface pass through said passageway to form a beam having a cross-sectional area much smaller than the area of said emitting surface.

3. An electron gun comprising a cathode having a hollow emitting surface conforming to the surface of an ellipsoid of rotation and having an axis of rotational symmetry perpendicular to said surface at the center thereof, an additional electrode positioned opposite the open end of said hollow emitting surface and having a passageway rotationally symmetrical about said axis, said passageway being convergent in the direction away from said cathode and having a toroidal surface at the cathode end and a conical surface at the opposite end, said two surfaces being joined longitudinally intermediate the ends of said passageway, and means for maintaining said additional electrode at a fixed potential relative to said cathode, whereby electrons released from said emitting surface of said cathode pass through said passageway to form a beam of high intensity and small cross-sectional area as compared with the area of said emitting surface.

4. An electron gun comprising a cathode having an emitting part and a non-emitting part, said emitting part having a hollow emitting surface conforming to the surface of an ellipsoid of rotation and having an axis of rotational symmetry perpendicular to said surface at the center thereof, said non-emitting part having means forming a truncated conical non-emitting surface the axis of which coincides with said axis of rotational symmetry, said truncated conical sur- I face having its maximum circumference equal to centered on said axis of rotational symmetry, an

additional electrode having rotational symmetry about said axis and having a maximum external diameter smaller than the diameter of said opening, said electrode extending through said plane surface forming means from a point inside the cathode space to a point outside the cathode space, a convergent passageway through said additional electrode having rotational symmetry about said axis and having its larger opening facing said hollow emitting surface, the surface of said passageway being toroidal at the larger end and conical at the smaller end with the two surfaces joining tangentially at a point intermediate the ends, and means maintaining the emitting and non-emitting parts of said cathode at a selected fixed electrical potential relative to said additional electrode, whereby electrons released by said emitting surface pass through said passageway to form an electron beam of much smaller cross-sectional area than the area of said emitting surface.

5. Apparatus as claimed in claim 4 in which the various elements have the following relative dimensions: depth of hollow emitting surface=6.15; maximum diameter of hollow emitting surface: 18.6; ratio of major to minor axis of said ellipsoid of rotation=1.3:l; apex angle of said truncated conical surface=22.5; altitude of said truncated conical surface=4.0; diameter of said opening: 14.0; maximum external diameter of said additional e1ectrode=12.0; radius of said toroidal surface=3.0; apex angle of conical surface of said passageway=40.0; and distance from plane containing edge of said hollow emitting surface to entrance to said passageway=3.15.

6. Apparatus as claimed in claim 4 in which the non-emitting part of said cathode is thermally insulated from the emitting part thereof and in which means are provided for heating said emitting part.

7. Apparatus as claimed in claim 4 in which the emitting part of said cathode is supported on an extension of the non-emitting part thereof by a plurality of pairs of oppositely disposed legs symmetrically arranged about said axis and inclined thereto, said legs being rigidly fixed at the ends to said emitting part and said extension and having such length that said emitting part when cold is in contact with said non-emitting part, whereby thermal expansion of said emitting part when heated spreads the distances between the fix points of said legs on said emitting part thus producing a slight separation of said emitting and non-emitting parts and preventing conduction of heat therebetween.

8. Apparatus as claimed in claim 2 in which the emitting part of said cathode is supported on an extension of the non-emitting part thereof by a plurality of pairs of oppositely disposed legs symmetricall arranged about said axis and inclined thereto, said legs being rigidly fixed at the ends to said emitting part and said extension and having such length that said emitting part when cold is in contact with said non-emitting part, whereby thermal expansion of said emitting part when heated spreads the distances between the fix points of said legs on said emitting part thus producing a slight separation of said emitting and non-emitting parts and preventing conduction of heat therebetween.

OSKAR HEIL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,979,392 Lubcke Nov. 6, 1934 2,079,163 Gardner et al. May 4, 1937 2,100,701 Schlesinger Nov. 30, 1937 2,146,365 Batchelor Feb. 7, 1939 2,174,853 Bowie Oct. 3, 1939 2,284,389 Hansen May 26, 1942 2,303,166 Laico Nov. 24, 1942 2,308,800 Anderson Jan. 19, 1943 

